1) Field of the Invention
The invention relates to the operation of a torch such as a plasma arc cutting torch and, more particularly, to a system and method for determining a continuity condition between the members of the torch and controlling the operation of the torch accordingly.
2) Description of Related Art
Plasma arc devices are commonly used for cutting and welding. One conventional plasma arc torch includes an electrode positioned within a nozzle. A pressurized gas is supplied to the torch and flows through the nozzle and proximate to the electrode, and an arc is established between the electrode and a workpiece. For example, according to one typical method for starting the torch, a pilot mode is first initiated by establishing an arc at a low current between the electrode and the nozzle. A metering system also delivers a flow of gas through the nozzle during the pilot mode. The torch is then switched from the pilot mode to a transfer or working mode by transferring the arc to the workpiece so that the arc extends between the electrode and the workpiece. The current of the arc is increased for the working mode, and the flow rate or type of the gas can also be adjusted. The arc ionizes the gas, and the resulting high temperature gas can be used for cutting or other welding operations. One such torch and a start-up operation are further described in U.S. Pat. No. 5,017,752, titled “Plasma arc torch starting process having separated generated flows of non-oxidizing and oxidizing gas,” issued May 21, 1991, which is assigned to the assignee of the present invention and the entirety of which is incorporated herein by reference.
In a conventional “blow-back” plasma arc torch, the nozzle is substantially fixed in position, and the electrode is configured to translate or adjust in a direction along the axis of the torch. The electrode is biased to a forward position by a spring or otherwise so that the electrode makes contact with the nozzle in a normal resting position. When the metering system provides a flow of gas to the torch, the flow of the gas adjusts the electrode in a direction away from the workpiece, thereby overcoming the spring and separating the electrode from the nozzle so that a pilot arc is established therebetween. In a “blow-forward” torch, the nozzle can instead be configured to adjust relative to the electrode so that the nozzle is adjusted in a forward direction by the flow of gas through the nozzle. In each case, a pilot arc can be established between the separated nozzle and electrode, and the arc can subsequently be transferred from the nozzle to the workpiece for a cutting or other welding operation.
In some cases, it is desirable to check or verify the continuity condition between the various components of the system. For example, generally in a blow-back or blow-forward torch, the nozzle and electrode should make contact when gas is not flowing through the nozzle, and the nozzle and electrode should not make contact when gas is flowing through the nozzle. A lack of continuity when no gas is flowing can indicate a missing, stuck, or otherwise faulty component. Similarly, continuity between the nozzle and electrode when the gas is flowing can indicate a stuck component or other improper electrical short. Such faults can prevent proper operation of the torch, e.g., by preventing a pilot arc from being established between the nozzle and electrode. In some cases, starting the torch when the components are in an improper configuration can damage the torch. For example, the electrode can be damaged if the torch is started while the workpiece is in electrical contact with the nozzle or electrode.
In some conventional torches, a continuity circuit is configured to determine if the electrode and the nozzle are in electrical contact so that starting of the torch can be prevented if the components are not separated. For example, the continuity circuit can connect the electrode to an output of a main power source that provides the current for the arcs during operation, and the circuit can detect a flow of the current from the electrode to the nozzle. However, this conventional continuity circuit does not separately indicate whether the nozzle is in electrical contact with the workpiece. Further, in some cases, the continuity circuit can be exposed to the high voltages and/or currents provided by the main power source for establishing the pilot and main arcs, thereby possibly damaging the continuity circuit.
Thus, there exists a need for an improved system and method for checking an operational condition of a gas torch and, in particular, determining the electrical continuity of the nozzle with the electrode or the workpiece. The system should be capable of checking the continuity of the torch without exposing the sensing circuit to an arc current of the main power source. Additionally, the system should be capable of controlling the operation of the torch according to the operational state of the torch.